This invention relates to the recording and reproduction of compressed image data, more particularly to the copying and editing of compressed image data.
In transmission and processing, information is subject to various forms of degradation, the reduction of which has been a principal object of research into transmission and processing methods. Digitization is one means of avoiding data degradation. Digital information processing and transmission are now in a period of rapid development due to advances in semiconductor technology.
What applies generally to information transmission and processing also applies particularly to the recording and reproduction of image signals. In the past, analog image signals were processed by analog methods, then recorded in analog form. Various types of image degradation occurred in both the signal-processing and signal-recording steps. When a recorded image was repeatedly copied or edited, the amount of degradation would increase each time.
To avoid this degradation, much recent work has been done on systems that process and record image signals using completely digital methods. In theory, an image can be digitized, then recorded and reproduced with substantially no degradation, and repeated copying and editing should cause substantially no additional degradation, so systems for digitally recording and reproducing image signals hold great future promise. A representative system of this type is the digital video tape recorder, hereinafter referred to as the digital VTR.
In practice, even a digital VTR has a finite signal-to-noise ratio, which leads to a certain number of errors in the reproduced data. When such errors occur, they tend to result in pixels with drastically incorrect color or brightness values. An error-correcting code capable of reducing the error rate to an acceptable level is therefore an essential requirement in a digital VTR.
Data compression is also essential. An NTSC analog video signal, for example, is commonly converted to a digital signal by sampling at three or four times the color subcarrier frequency (f.sub.SC) of 3.58 MHz. If the sampling rate is 3f.sub.SC with eight bits per sample, the resulting data rate is 86 megabits per second (8.times.3.times.3.58 MHz), requiring about twenty times the bandwidth of the original analog signal. While digitization of an image signal can prevent degradation, it vastly increases the amount of information to be transmitted or recorded. To reduce the data rate, error-correcting encoding is therefore preceded by a separate compressive encoding step.
In a digital VTR, a third type of encoding, referred to as channel encoding, is also necessary in order to record the data on magnetic tape. A digital VTR accordingly has three encoders: a source encoder that digitizes and compresses the video signal; an error-correcting encoder that adds an error-correcting code to the compressed data; and a channel encoder that converts the output of the error-correcting encoder to a form suitable for magnetic recording. For reproducing recorded images, the digital VTR also has three decoders: a channel decoder that converts the signal read from the magnetic tape to a data signal; an error-correcting decoder that corrects errors in the data signal; and a source decoder that decompresses the corrected data signal and converts it back to an analog video signal.
The error-correcting encoder and decoder are necessarily limited in their error-correcting capability; uncorrectable errors occasionally occur. In copying and editing, if the reproduced data were to be decompressed and converted back to analog form, then redigitized and recompressed before being recorded again, the effect of uncorrectable errors would be magnified, and errors would tend to propagate into previously correct data, causing serious image degradation. This problem has been addressed in the prior art (e.g. Japanese Patent Kokai Publication 133573/1985) by copying and editing compressed data.
More specifically, when a tape is copied from machine A to machine B, the output of the error-correcting decoder in machine A (including both the compressed image data and error-correcting code) is fed to the channel encoder in machine B, bypassing the source decoder in machine A and the source encoder and error-correcting encoder in machine B. Any uncorrectable errors are therefore left as is, without being magnified by decompression and recompression.
Nevertheless, this scheme does not completely eliminate error propagation effects. Once an uncorrectable error occurs in the image data, it tends to breed further errors in future copies and edits by impairing the error-correcting ability of the error-correcting decoder, so that errors that would normally be correctable become uncorrectable. As the number of uncorrected errors grows, the error-correcting decoder also becomes prone to make mistakes by overlooking errors; or by miscorrecting errors, thereby introducing new errors itself. These mistakes moreover cause the error-correcting decoder to misinform the source decoder as to the presence or absence of errors. Thus when image data are repeatedly copied, errors can still propagate to the point where significant image degradation occurs.